1. Field of the Invention
The present invention relates to the field of hydrodynamic bearing spindle motors for disc drive data storage devices and, more particularly, to a motor having a lubricant with an additive and/or a combination of additives or lubricants for reducing the temperature dependency of the lubricant's viscosity.
2. Description of the Related Art
Disc drive data storage devices, known as “Winchester” type disc drives, are well-known in the industry. In a Winchester disc drive, digital data is written to and read from a thin layer of magnetizable material on the surface of rotating discs. Write and read operations are performed through a transducer that is carried in a slider body. The slider and transducer are sometimes collectively referred to as a head, and typically a single head is associated with each disc surface. The heads are selectively moved under the control of electronic circuitry to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. Each slider body includes a self-acting air bearing surface. As the disc rotates, the disc drags air beneath the air bearing surface, which develops a lifting force that causes the slider to lift and fly several microinches above the disc surface.
In the current generation of disc drive products, the most commonly used type of actuator is a rotary moving coil actuator. The discs themselves are typically mounted in a “stack” on the hub structure of a brushless DC spindle motor. The rotational speed of the spindle motor is precisely controlled by motor drive circuitry, which controls both the timing and the power of commutation signals directed to the stator windings of the motor. Typical spindle motor speeds have been in the range of 3600 RPM. Current technology has increased spindle motor speeds to 7200 RPM, 10,000 RPM and above.
One of the principal sources of noise in disc drive data storage devices is the spindle motor. Disc drive manufacturers have recently begun looking at replacing conventional ball or roller bearings in spindle motors with “hydro” bearings, such as hydrodynamic or hydrostatic bearings. A hydro bearing relies on a fluid film which separates the bearing surfaces and is therefore much quieter and in general has lower vibrations than conventional ball bearings. A hydrodynamic bearing is a self-pumping bearing that generates a pressure internally to maintain the fluid film separation. A hydrostatic bearing requires an external pressurized fluid source to maintain the fluid separation. Relative motion between the bearing surfaces in a hydro bearing causes a shear element that occurs entirely within the fluid film such that no contact between the bearing surfaces occurs.
In a hydro bearing, a lubricating fluid provides a bearing surface between a stationary member of the housing and a rotating member of the disc hub. Typical lubricants include oil or ferromagnetic fluids. Hydro bearings spread the bearing surface over a larger surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface decreases wobble or run-out between the rotating and fixed members.
Viscosity and anti-wear performance are important considerations in miniature hydro bearings for disc drives. The lubrication properties that must be controlled, and the degree of control that must be obtained, are unique to these bearings. Viscosity determines power dissipation and bearing stiffness, which should be relatively constant over various operating conditions. In general, viscosity of the lubricant decreases with an increase in temperature. In high-speed, high-temperature applications, reduced viscosity of the lubricant can result in an unexpected touch down of the mating surfaces of the hydro bearings because of the loss of stiffness and may cause premature failure of the disc drive.
Therefore, there exists a need in the art for a hydrodynamic fluid bearing having lubricants exhibiting reduced viscosity temperature dependency in disc drive applications.